That’s sort of part of the joke. ACM is the “Association for Computing Machinery” one of the biggest and oldest and nerdiest computer clubs.
ACM hosts all kinds of SIGs (special interest groups) - clubs dedicated to interest in (sometimes deeply esoteric) aspects of computer science. For a few of them, hand-coding a new specialized fast floating point calculation code during a contest could easily come up.
So there’s technically a situation where lying to a peer, on purpose, in a particularly mean context (such as a competition) about a floating point number computation - could actually get someone kicked out of ACM.
Edit: Additonal context that helps the joke - my experiences with ACM have all been super chill , relaxed and friendly. So “I got kicked out of ACM” is also a “you did what?!” setup for the joke.